We have extensively characterized a non-conditional mutant of B-tropic MuLV. This mutant contains altered pol precursors and packages a smaller pol molecule in virions. This molecule retains some catalytic and immunological reactivity. When cells expressing Fv-1 restriction are infected with a single particle of restricted MuLV, they become transiently permissive to infection by other virus particles of the restricted tropism. This phenomenon is termed "abrogation" of Fv-1 restriction. Abrogation does not require cellular or viral DNA synthesis but does require intact viral RNA. Thus, viral RNA functions in abrogation not as a template for DNA synthesis but by some other biochemical pathway. This new pathway may reveal new steps early in viral infection.